Bringing any type of drug into or out of a country is illegal in most places, so don’t bring them with you when traveling (or at least make sure it’s legal first).
Driving under influence of drugs is also illegal in most places, and for good reason. Do not drive under influence to avoid any problems.
Many places do not allow the use of drugs in public, even when they are legalized for recreational use. Make sure it is legal before lighting a joint in a public place.
In Washington DC, medical marijuana was made legal decades ago and in November 2014, the legalization of cannabis for recreational use followed, when residents passed the Minimal amounts of marijuana for personal use initiative.
Even though recreational use of marijuana was legalized, it failed to address the sale of it. Residents mostly grew it or got it from others who grew it themselves, which was also made legal with the passing of the Recreational use of marijuana initiative. While there is still the unresolved issue about one aspect of the sale, it is perfectly legal to sell marijuana paraphernalia.
In November 2016 it was made legal for visitors to purchase cannabis products for medical use with the medical marijuana card from their own home state.
As with all other states, there is a problem with cannabis not being legal on federal property yet, which makes a lot of property in Washington DC excluded from the legalization.